GB2377026A

GB2377026A - Electrically addressable electrochemical cell array

Info

Publication number: GB2377026A
Application number: GB0116057A
Authority: GB
Inventors: Mino Green
Original assignee: Imperial College Innovations Ltd
Current assignee: Ip2ipo Innovations Ltd
Priority date: 2001-06-29
Filing date: 2001-06-29
Publication date: 2002-12-31
Also published as: US20040194295A1; WO2003002249A2; EP1438127A2; GB0116057D0; WO2003002249A3; AU2002345162A1

Abstract

The array is formed, on an insulating substrate 2 by, forming a set of parallel conductor strips 4, forming an insulating layer on the strips, forming a second set of parallel strips 6 at right angles to the first strips so as to form crossover regions, depositing a very thin film of highly soluble solid on the upper surface, exposing the film to solvent vapour under controlled conditions so that the film reorganises into an array of discrete hemispherical islands 8 over the surface with a plurality of islands overlaying each crossover region, depositing a resist material over the whole surface, removing the islands plus overlaying resist to leave a resist layer with an array of holes, and etching so as to form wells extending through both sets of strips at the crossover regions. Substances in the cells in the array can be reacting with specific reagents by contacting the whole array with an electrolyte, connecting an electrical supply to metal strips defining cells which are to be protected from reaction, and electrolysing the electrolyte to form a gas bubble over material in those cells.

Description

"Nano-Electrochemical Cells" This invention relates to a method of fabricating nanometer sized electrochemical cells in a multilayer substrate. In particular, it is concerned with a method of fabricating an electrically addressable array of cells that will find application in combinatorial synthesis and chemical analysis.

In the process of combinatorial synthesis, it is required to experimentally combine various different chemicals, in large numbers of alternative combinations, in order to investigate possible useful compounds that may result. Consequently, it is desirable to provide a method of at least partially automating the process whereby the different substances are allowed to come into contact with one another.

Similarly, in chemical analysis, it is often necessary to experimentally apply a number of different reagents to a compound under test, and this process could be considerably speeded up, if it were possible to automatically control the combination of a series of pairs of substances, in a suitable array of receptacles.

In our copending International patent application, publication no. 01/13414, we have described a method of island lithography for producing an array of"wells"in a substrate, which comprises the steps of: a) depositing a very thin film of a highly soluble solid onto a flat hydrophilic substrate; b) exposing the film to solvent vapour under controlled conditions so that the film reorganises into an array of discrete hemispherical islands on the surface; c) depositing a film of a suitable resist material over the whole surface; d) removing the hemispherical structures together with their coating of resist leaving a resist layer with an array of holes corresponding to the islands; and e) subjecting the resulting structure to a suitable etching process so as to form a well at the position of each hole.

The highly soluble solid may be a salt such as cesium chloride, In which case the solvent used will be water. Preferably the resist material IS aluminium which is vapour-deposited, and In a preferred embodiment the removal of the coated hemispherical structures IS achieved by submerging the structure in an ultrasonic agitation bath filled with solvent that has the effect of dissolving the islands and thus removing the thin layer of material in which they were coated, leaving a perforated film over the rest of the substrate, namely covering the"ocean"area in which the islands are located. This process step is known as a "lift-off'process. This perforated film whose holes correspond to the now removed islands can act as a resist in an etching process.

In the above mentioned application, the method is described primarily, as a means of fabricating semiconductor devices in a silicon substrate, but it is also applicable to other kinds of substrates, and may, for example, be utilised in order to form wells in a suitable multilayer structure of electrodes, so as to provide a multiplexed array of electrochemical cells.

Accordingly, the present invention provides a method of forming an array of electrically addressable cells, comprising the steps of: (a) forming a set of parallel conductor strips, extending in a first direction, on an insulating substrate; (b) forming an insulating layer superimposed on the first series of parallel strips; (c) forming a second set of parallel conductor strips, extending in a direction at right angles to the first direction, superimposed on the insulating layer so as to form crossover regions between the strips, (d) depositing a very thin film of a highly soluble solid onto the upper surface of the structure;

e) exposing the film to solvent vapour under controlled conditions so that the film reorganises into an array of discrete hemispherical islands distnbuted over the surface with a plurality of islands overlying each crossover region; f) depositing a film of a suitable resist material over the whole surface; g) removing the hemispherical structures together with their coating of resist leaving a resist layer with an array of holes corresponding to the islands ; and h) subjecting the resulting structure to a suitable etching process so as to form a well at the position of each hole ; whereby the wells extend through both sets of conductive strips at the crossover regions.

According to a further feature of the invention, there is provided a method of selectively reacting a substance with a series of different reagents, comprising the steps of: (a) introducing the reactive substance under investigation, into an array of cells formed in a matrix by the method described above, (b) applying an electrolyte to the array, in order to fill all of the cells with electrolyte; (c) connecting an electrical supply to at least one of the metal strips of each set of the array, so as to address the corresponding group of cells at each crossover region; whereby the water of the electrolyte is electrolysed to produce a gas bubble at each cell which protects the chemically reactive area in the lower region of the cell ; and (d) applying a reagent onto the array of cells so that it can only react with the cells which have not been addressed.

Preferably, the side walls of the cells are treated so as to be hydrophilic.

One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, In which: Figure 1 is a diagrammatic view of the layer structure of the device according to the invention ;

Figure 2 ts a plan view of a multiplexed array of cells, Figure 3a IS a vertical cross section through the multilayer cell structure of Figure

1 ; Figure 3b is a cross section corresponding to that of Figure 3a, and showing wells filled with electrolyte ; and Figure 3c illustrates the well structure with a gas column isolating material at the bottom of the well, from the electrolytic reagent.

Figure 1 illustrates an"exploded"view of the multiplexed cell structure, which is formed on a substrate 2 which may be, for example, silicon with an insulating layer of Si02. A set of parallel conductive metal strips 4 is formed on the substrate, for example by a suitable photolithographic process, upon which is superimposed a further silica insulating layer such as Si02 (not shown in Figure 1), and a further set of parallel conductive strips 6 is then formed on the silica insulating layer, extending at right angles to the first set of strips 4.

Accordingly, this provides a rectangular array of conductors, shown in plan view in Figure 2, and it will be appreciated that the region at the interstices of the strips can therefore be addressed electrically, by applying a suitable potential across one of the strips of each set. As shown in Figure 2, a voltage +V has been applied to three of the strips 6, while a voltage of-V has been applied to two of the strips 4, thereby subjecting six shaded regions to the array to the corresponding difference in potential.

As indicated by the pattern A of apertures in the electrode strips 4 and 6 in Figure 1, the structure has also been formed overall with a large number of wells by the process of"island lithography"described in more detail above. Figure 3 illustrates an enlarged view of the structure, at the Intersection of two of the strips 4 and 6, to show how the cells are utilised in practice. Figure 3a illustrates the cross section of the relevant region in more detail. An Insulating layer of silicon dioxide 10, having a depth of about 20 nanometers, IS formed on the silicon substrate 2, and the strips 4, which in the example

are made of gold, are deposited to a thickness of approximately 30 nanometers on the insulator 10. A further insulating layer 12 of Si02, having a depth of approximately 50 nanometers IS superimposed on the electrodes 4.

The process of island lithography is then applied to the resulting structure, which forms a number of wells at each intersection, only one of which is shown in Figure 3b. In this example, the metal conductive strips are approximately one micron wide, and the diameter of each well is about 50 nanometers and has a depth of 120 nanometers. The process forms cells at a density of about 100 per sq. micron.

The structure may be arranged to provide an electrochemical"shutter"for a chemically active area 12, in the bottom of the well 14, in the following way. If a suitable potential is applied across the relevant wells, as described above with reference to Figure 2, an electrolyte 16 can be applied to the upper surface of the structure, and as illustrated in Figure 3b, will enter cells 14, where no electrical potential has been applied.

However, as shown in Figure 3c, where an electrical potential has been applied, the water of the electrolyte will be electrolysed to yield oxygen and hydrogen, which in the normal way, can dissolve in the electrolyte. However, if they are generated (particularly from the lower electrode 4) faster than they can dissolve, then a gas bubble will form and grow to the shape shown in Figure 3c. The result of this is the formation of a gas column, with a bubble 18 at its upper end, which protects the chemically reactive area 12 in the lower region of the well, from the chemical reaction.

A sustaining electroylsis current will flow in the absorbed water multi layer that will result from the water saturated atmosphere in the bubble. The side walls of the well are preferably treated so as to be hydrophilic.

"Shutter"characteristics An approximate idea of the sustaining current required at a single well can be obtained by making use of an early paper"On the stability of gas bubbles in liquid-gas solutIon" (P. S. Epstein and M. S. Plesset, J. Chem. Phys., 18 (1950) 1505-1509). Hence

the authors deduce the relation for the time, T, for a bubble of gas (formed Instantaneously) of initial radius Ro to completely dissolve In water, namely,

Where a = K (C C1)/Q

K is the diffusion coefficient of H2 or 02 in water ; Cs is the saturation solubility of the gas in water C1 is the initial concentration of the gas in the water ; and Q is density of gas in the bubble. The pressure of gas in the bubble, p, r, will be related the pressure of gas outside the bubble, pout, the bubble radius, R, and the surface tension,'1 (, of the gas/solution interface given by the equation (first proposed by Laplace),

Example γ: surface tension, Nom-1 : water at 220C is 7.3 x 10-2Nm-' Atmospheric pressure: 1 atmos = 760 Torr (mmHg); 105Pa (Nm-2) Henry's Law const. : Khydrogen = 5.34 x Torr and Oxygen= 3.30 x 107Torr Diffusion coefficient of hydrogen molecules in water: m2sec-1 = 5x10-9m2sec-1 e: charge on electron = 1.6 x 1-0-19 Coulombs.

N: Avagadro's number = 6.02 x 1023 particles per mol. n (H20) : number of moles In 1 kg of water = 55.5 Now suppose that we take the radius of the bubble shown in the diagram as 40nm (400 ), this is sitting on top of a 25nm radius well. The external gas pressure will be 0.2 atmosphere for 02 (i. e. 0.2 x 760Torr), but zero for hydrogen, and the term 2 r ;/R == 2 x 7.3 x 10-2/4 x 10-8 == 3.65 x 106 Pa = 36.5 atmospheres = 2.74 x 104 Torr, and clearly we may neglect pout : this IS a high internal bubble pressure.

The saturation solubility of gas (dependent upon pressure) is given, using Henry's Law, as x = p/K, where x IS the mole fraction of solute and p is its partial pressure, and K is a materials specific constant. In our case of dilute solutions we may write that the number of moles of gas, n (gas), dissolved in 1 kg of water is,

K IS given above. So for hydrogen:

This is a molality that we approximate ea'3ily to a molarity, so n (gas) will be in mol/litre.

At 36.5 x 760 Torr (36.5 atmosphres) we have; n (Hz = 0.029 mol/litre : and n (02) = 0. 046 mol/litre.

Before calculating the bubble time T above we need ex, in which we take K as

5 x 1O. 9mz/sec ; and Cg/Q is 0. 029/ (36. 5-22. 4) = 0. 018. So that ex = 5 x 10. 9 x 0. 018 = 9 x 10'1 m2/sec. Thus T is 1. 6 x 10-15/9 x 10-11 sec. = 1. 8x 10-3 sec.

The amount of material in moles in a bubble is about (4/3) TTRo3 x (36. 5-22. 4) x

103 = 4. 19 x (4 X 10) 3 x1. 63x 103 10'9 moles of Hz. This corresponds to a requirement for discharge of, 2 x 4. 37 X 10. x 6. 02 X 1023 = 5 x 105 electrons per bubble. This would correspond to 5 x 105/1. 9 X 10. = 2. 6 x 108 electrons per second. If the cathode area is 2TTRwh (Rw is the well radius and h is the thickness if the cathode layer) = 6. 28 x 2. 5 x 10-8 x 3 x 10. == 4. 7 x 10. 15 mZ : then the sustaining current density is about 5. 5 x 1022 electrons m' = 8850 Amps m'. Or with a well coverage of about 20% -2 ; MilliaMpS MM-2the current densities are 0. 18 Amps cm' ; 1. 8 milliamps mm'z ; or 1. 8 x 10. Amps/sq. micron. This is an upper limit estimate.

A modest current density will be able to sustain a gas shutter over the bottom of the well area, and on turning the current off the shutter will go in the order of a millisecond.

Multiplexing The equilibrium discharge potential for the electrolysis of water is 1.23 volts. The current/voltage characteristics, I/V, of the anode and cathode are both non-linear and best described by an equation of the form,

where to is the exchange current s is a constant depending on the mechanism of discharge but often equal to 2 and e/kT has its usual meaning. This equation is similar in form to that of a forward biased diode. It is this type of relation that makes multiplexing possible. The basic notion is that a voltage Vth is required to give the current density for gas shutter formation while Vth/2 will be well below 1.23 volts and so give no discharge at all. Typical anodic to values are 10-10 Amps cam-2.

Figure 2 shows the notions of a multiplexed array. Note that the metal layers are fabricated into strips (say 1 micron wide) so that they constitute a matrix array N x M.

It should be possible, because of the non-linearity of the IN characteristic, to select a particular line open while shuttering off all the other lines. This, xth, line will be treated chemically and then closed and the next, (x+1) th line opened and treated, and so forth. So we have N different lines of material attached in the wells. Now we can shutter off-close-all the columns except the y" column and expose it to a particular reagent, now we have all the N different rows reacted with a particular reagent In their myth column, and so forth until we have a matnx N x M of pixellated reaction products, having carried out N + M operations.

This simple, line-at-a-time multiplexing, does not allow us to shutter off all the pixels except one. It does allow us to shutter off one pixel and open all the others. A more elaborate multiplexing scheme could be envisaged that took advantage of an underlying semiconductor substrate, e. g. silicon, that could be processed into an active matrix array.

The structure of the present invention has been devised in response to the need for array synthesis and analysis that is particularly relevant to drug discovery and development. It does not directly address the question of chemical identification or chemical release at each pixel. However the well defined matrix array structure will readily lend itself to e. g. scanning analytical tools.

Claims

1 A method of forming an array of electrically addressable cells, comprising the steps of : (a) forming a set of parallel conductor strips, extending in a first direction, on an insulating substrate; (b) forming an insulating layer superimposed on the first series of parallel strips; (c) forming a second set of parallel conductor strips, extending in a direction at right angles to the first direction, superimposed on the insulating layer so as to form crossover regions between the strips, (d) depositing a very thin film of a highly soluble solid onto the upper surface of the structure; e) exposing the film to solvent vapour under controlled conditions so that the film reorganises into an array of discrete hemispherical islands distributed over the surface with a plurality of islands overlying each crossover region; f) depositing a film of a suitable resist material over the whole surface; g) removing the hemispherical structures together with their coating of resist leaving a resist layer with an array of holes corresponding to the islands; and h) subjecting the resulting structure to a suitable etching process so as to form a well at the position of each hole ; whereby a matrix-addressable group of wells, extending through both sets of conductive strips, is formed at each crossover region.

2. A method according to claim 1 in which the substrate is silicon with an insulating layer of Si02.

3. A method according to claim 1 or claim 2 in which the conductor strips are gold.

4. A method according to any of claims 1 to 3 in which the highly soluble solid is a salt.

5. A method according to claim 4 in which the salt IS cesium chloride.

6. A method according to claim 4 or claim 5 in which the solvent is water.

7. A method according to any preceding claim in which the resist matenal is vapour-deposited aluminium.

8. An electrically addressable array of group of cells comprising wells formed by a method according to any of claims 1 to 7.

9. A method of selectively reacting a substance with a series of different reagents using an electrically addressable array of cells according to claim 8, comprising the steps of : (a) introducing the substance into the array of cells ; (b) applying an electrolyte to the array, in order to fill all of the cells with electrolyte ; (c) connecting an electrical supply to at least one of the metal strips of each set of the array, so as to address the corresponding group of cells at each crossover region ; whereby the water of the electrolyte is electrolysed to produce a gas bubble at each cell which protects the chemically reactive area in the lower region of the cells ; and (d) applying a reagent onto the array of cells so that it can only react with the cells which have not been addressed.

10. A method of forming a multiplexed array of electrochemical cells substantially as herein described with reference to the accompanying drawings.

11. An electrically addressable array of cells substantially as herein described with reference to the accompanying drawings.

12. A method of selectively reacting a substance with a series of different reagents substantially as herein described.